I have been on both in Y (E-). Food choices are better and include western on UA. Also, if you're a drinker, UA offers free beverages in Y on Asia flights (TPAC+NRT). CA offers a choice of juices, coke, sprite?, and hot drinks, all out of a large bottle (ie, no whole cans or stuff like diet).
YMMV, but on the UA flights, they had sandwiches and snacks in the galley. I don't recall getting up to check on the CA flight.
However, flying on the 744-combi was a great experience. Between what I mentioned, a slight dislike of CA service, and reduced UA miles, I can't see a reason I'd personally fly CA longhaul again.